This invention relates to rotary electric machines which are wound in the pole amplitude modulation scheme of stator coil connections and more particularly to the use of controlled switching devices with such machines to permit continuous adjustable speed operation between two fixed operating speeds characteristic of the pole amplitude modulation operating points.
Alternating current machines which provide alternative first and second pole numbers by a method of phase winding switching known as pole amplitude modulation are well known and have been described in various patents and publications. For example, several embodiments of pole changing machines have been described in two papers by G. H. Rawcliffe et al., a first entitled "Induction-Motor Speed-Changing by Pole-Amplitude Modulation" in the Proceedings of the Institution of Electrical Engineers, Volume 105, Part A, No. 22, August 1958, and a second entitled "Speed-Changing Induction Motors, Further Developments in Pole-Amplitude Modulation" in the Proceedings of the Institution of Electrical Engineers, Volume 107, Part A, No. 36, December 1960. Pole amplitude modulated machines generally include a first and second set of terminals for connection to an external AC power source and a stator winding alternatively connectable to the first and second sets of terminals and wound to form a first number of poles between the first set of terminals and a second number of poles between the second set of terminals. The operating speed of the machine can be selected by connecting the external power source to the appropriate set of terminals. The efficient range of operation of these machines is limited to a few percent slip from the synchronous speed for the pole number for which they are connected.
The present invention combines a conventional pole amplitude modulated machine and power electronic control circuitry at the stator terminals to give a constant efficiency continuously adjustable speed operation between the two fundamental speed changing points characteristic of the conventional pole amplitude modulated winding lay-out, without variable frequency control. For an induction machine, the invention retains the conventional cage rotor and a uniform slot pitch and turns per coil around the stator periphery.